This chapter describes JavaScript expressions and operators, including assignment, comparison, arithmetic, bitwise, logical, string, and special operators.

Expressions

An expression is any valid unit of code that resolves to a value.

Conceptually, there are two types of expressions: those that assign a value to a variable, and those that simply have a value. For example, the expression x = 7 is an expression that assigns x the value seven. This expression itself evaluates to seven. Such expressions use assignment operators. On the other hand, the expression 3 + 4 simply evaluates to seven; it does not perform an assignment. The operators used in such expressions are referred to simply as operators.

Operators

JavaScript has both binary and unary operators, and one special ternary operator, the conditional operator. A binary operator requires two operands, one before the operator and one after the operator:

operand1 operator operand2

For example, 3+4 or x*y.

A unary operator requires a single operand, either before or after the operator:

operator operand

or

operand operator

For example, x++ or ++x.

Assignment operators

An assignment operator assigns a value to its left operand based on the value of its right operand. The basic assignment operator is equal (=), which assigns the value of its right operand to its left operand. That is, x = y assigns the value of y to x.

The other assignment operators are shorthand for standard operations, as shown in the following table.

Comparison operators

This seems to me kind of poorly explained, mostly the difference between "==" and "==="... A comparison operator compares its operands and returns a logical value based on whether the comparison is true. The operands can be numerical, string, logical, or object values. Strings are compared based on standard lexicographical ordering, using Unicode values. In most cases, if the two operands are not of the same type, JavaScript attempts to convert the operands to an appropriate type for the comparison. (The sole exceptions to this rule are === and !==, which perform "strict" equality and inequality and which do not attempt to convert the operands to compatible types before checking equality.) This generally results in a numerical comparison being performed. The following table describes the comparison operators, assuming the following code:

var var1 = 3, var2 = 4;

Arithmetic operators

Arithmetic operators take numerical values (either literals or variables) as their operands and return a single numerical value. The standard arithmetic operators are addition (+), subtraction (-), multiplication (*), and division (/). These operators work as they do in most other programming languages when used with floating point numbers (in particular, note that division by zero produces NaN). For example:

1 / 2; /* prints 0.5 */
1 / 2 == 1.0 / 2.0; /* also this is true */

In addition, JavaScript provides the arithmetic operators listed in the following table.

Bitwise operators

Bitwise operators treat their operands as a set of 32 bits (zeros and ones), rather than as decimal, hexadecimal, or octal numbers. For example, the decimal number nine has a binary representation of 1001. Bitwise operators perform their operations on such binary representations, but they return standard JavaScript numerical values.

The following table summarizes JavaScript's bitwise operators.

Bitwise logical operators

Conceptually, the bitwise logical operators work as follows:

• The operands are converted to thirty-two-bit integers and expressed by a series of bits (zeros and ones).
• Each bit in the first operand is paired with the corresponding bit in the second operand: first bit to first bit, second bit to second bit, and so on.
• The operator is applied to each pair of bits, and the result is constructed bitwise.

For example, the binary representation of nine is 1001, and the binary representation of fifteen is 1111. So, when the bitwise operators are applied to these values, the results are as follows:

Note that all 32 bits are inverted using the Bitwise NOT operator, and that values with the most significant (left-most) bit set to 1 represent negative numbers (two's-complement representation).

Bitwise shift operators

The bitwise shift operators take two operands: the first is a quantity to be shifted, and the second specifies the number of bit positions by which the first operand is to be shifted. The direction of the shift operation is controlled by the operator used.

Shift operators convert their operands to thirty-two-bit integers and return a result of the same type as the left operand.

The shift operators are listed in the following table.

Logical operators

Logical operators are typically used with Boolean (logical) values; when they are, they return a Boolean value. However, the && and || operators actually return the value of one of the specified operands, so if these operators are used with non-Boolean values, they may return a non-Boolean value. The logical operators are described in the following table.

Examples of expressions that can be converted to false are those that evaluate to null, 0, the empty string (""), or undefined.

The following code shows examples of the && (logical AND) operator.

var a1 =  true && true;     // t && t returns true
var a2 =  true && false;    // t && f returns false
var a3 = false && true;     // f && t returns false
var a4 = false && (3 == 4); // f && f returns false
var a5 = "Cat" && "Dog";    // t && t returns Dog
var a6 = false && "Cat";    // f && t returns false
var a7 = "Cat" && false;    // t && f returns false

The following code shows examples of the || (logical OR) operator.

var o1 =  true || true;     // t || t returns true
var o2 = false || true;     // f || t returns true
var o3 =  true || false;    // t || f returns true
var o4 = false || (3 == 4); // f || f returns false
var o5 = "Cat" || "Dog";    // t || t returns Cat
var o6 = false || "Cat";    // f || t returns Cat
var o7 = "Cat" || false;    // t || f returns Cat

The following code shows examples of the ! (logical NOT) operator.

var n1 = !true;  // !t returns false
var n2 = !false; // !f returns true
var n3 = !"Cat"; // !t returns false

Short-circuit evaluation

As logical expressions are evaluated left to right, they are tested for possible "short-circuit" evaluation using the following rules:

• false && anything is short-circuit evaluated to false.
• true || anything is short-circuit evaluated to true.

The rules of logic guarantee that these evaluations are always correct. Note that the anything part of the above expressions is not evaluated, so any side effects of doing so do not take effect.

String operators

In addition to the comparison operators, which can be used on string values, the concatenation operator (+) concatenates two string values together, returning another string that is the union of the two operand strings. For example, "my " + "string" returns the string "my string".

The shorthand assignment operator += can also be used to concatenate strings. For example, if the variable mystring has the value "alpha", then the expression mystring += "bet" evaluates to "alphabet" and assigns this value to mystring.

Special operators

Conditional operator

The conditional operator is the only JavaScript operator that takes three operands. The operator can have one of two values based on a condition. The syntax is:

condition ? val1 : val2

If condition is true, the operator has the value of val1. Otherwise it has the value of val2. You can use the conditional operator anywhere you would use a standard operator.

For example,

var status = (age >= 18) ? "adult" : "minor";

This statement assigns the value "adult" to the variable status if age is eighteen or more. Otherwise, it assigns the value "minor" to status.

Comma operator

The comma operator (,) simply evaluates both of its operands and returns the value of the second operand. This operator is primarily used inside a for loop, to allow multiple variables to be updated each time through the loop.

delete

The delete operator deletes an object, an object's property, or an element at a specified index in an array. The syntax is:

delete objectName;
delete objectName.property;
delete objectName[index];
delete property; // legal only within a with statement

where objectName is the name of an object, property is an existing property, and index is an integer representing the location of an element in an array.

The fourth form is legal only within a with statement, to delete a property from an object.

You can use the delete operator to delete variables declared implicitly but not those declared with the var statement.

If the delete operator succeeds, it sets the property or element to undefined. The delete operator returns true if the operation is possible; it returns false if the operation is not possible.

x = 42;
var y = 43;
myobj = new Number();
myobj.h = 4;    // create property h
delete x;       // returns true (can delete if declared
		// implicitly)
delete y;       // returns false (cannot delete
		// if declared with var)
delete Math.PI; // returns false (cannot delete 
		// predefined properties)
delete myobj.h; // returns true (can delete user-defined
		// properties)
delete myobj;   // returns true (can delete if 
		// declared implicitly)

Deleting array elements

When you delete an array element, the array length is not affected. For example, if you delete a[3], a[4] is still a[4] and a[3] is undefined.

When the delete operator removes an array element, that element is no longer in the array. In the following example, trees[3] is removed with delete. However, trees[3] is still addressable and returns undefined.

var trees = new Array("redwood", "bay", "cedar", 
"oak", "maple");

delete trees[3];
if (3 in trees) {
  // this does not get executed
}

If you want an array element to exist but have an undefined value, use the undefined keyword instead of the delete operator. In the following example, trees[3] is assigned the value undefined, but the array element still exists:

var trees = new Array("redwood", "bay", "cedar",
 "oak", "maple");
trees[3] = undefined;
if (3 in trees) {
  // this gets executed
}

in

The in operator returns true if the specified property is in the specified object. The syntax is:

propNameOrNumber in objectName

where propNameOrNumber is a string or numeric expression representing a property name or array index, and objectName is the name of an object.

The following examples show some uses of the in operator.

// Arrays
var trees = new Array("redwood", "bay", "cedar", 
"oak", "maple");
0 in trees;        // returns true
3 in trees;        // returns true
6 in trees;        // returns false
"bay" in trees;    // returns false (you must specify 
                   // the index number, not the value
		   // at that index)
"length" in trees; // returns true

// Predefined objects
"PI" in Math;          // returns true
var myString = new String("coral");
"length" in myString;  // returns true

// Custom objects
var mycar = {make: "Honda", model: "Accord", year: 1998};
"make" in mycar;  // returns true
"model" in mycar; // returns true

instanceof

The instanceof operator returns true if the specified object is of the specified object type. The syntax is:

objectName instanceof objectType

where objectName is the name of the object to compare to objectType, and objectType is an object type, such as Date or Array.

Use instanceof when you need to confirm the type of an object at runtime. For example, when catching exceptions, you can branch to different exception-handling code depending on the type of exception thrown.

For example, the following code uses instanceof to determine whether theDay is a Date object. Because theDay is a Date object, the statements in the if statement execute.

var theDay = new Date(1995, 12, 17);
if (theDay instanceof Date) {
  // statements to execute
}

new

You can use the new operator to create an instance of a user-defined object type or of one of the predefined object types Array, Boolean, Date, Function, Image, Number, Object, Option, RegExp, or String. On the server, you can also use it with DbPool, Lock, File, or SendMail. Use new as follows:

var objectName=new objectType([param1, param2, ...]);

See the new operator page in the Core JavaScript Reference for more information.

typeof

The typeof operator is used in either of the following ways:

typeof operand

typeof (operand)

The typeof operator returns a string indicating the type of the unevaluated operand. operand is the string, variable, keyword, or object for which the type is to be returned. The parentheses are optional.

Suppose you define the following variables:

var myFun = new Function("5 + 2");
var shape = "round";
var size = 1;
var today = new Date();

The typeof operator returns the following results for these variables:

typeof myFun;     // returns "function"
typeof shape;     // returns "string"
typeof size;      // returns "number"
typeof today;     // returns "object"
typeof dontExist; // returns "undefined"

For the keywords true and null, the typeof operator returns the following results:

typeof true; // returns "boolean"
typeof null; // returns "object"

For a number or string, the typeof operator returns the following results:

typeof 62;            // returns "number"
typeof 'Hello world'; // returns "string"

For property values, the typeof operator returns the type of value the property contains:

typeof document.lastModified; // returns "string"
typeof window.length;         // returns "number"
typeof Math.LN2;              // returns "number"

For methods and functions, the typeof operator returns results as follows:

typeof blur;        // returns "function"
typeof eval;        // returns "function"
typeof parseInt;    // returns "function"
typeof shape.split; // returns "function"

For predefined objects, the typeof operator returns results as follows:

typeof Date;     // returns "function"
typeof Function; // returns "function"
typeof Math;     // returns "object"
typeof Option;   // returns "function"
typeof String;   // returns "function"

Operator precedence

The precedence of operators determines the order they are applied when evaluating an expression. You can override operator precedence by using parentheses.

The following table describes the precedence of operators, from highest to lowest.

In accordance with relevant discussion, this table was reversed to list operators in decreasing order of priority.